Whatever you think about John Terry, nobody can deny he has been a formidable
defender and a magnificent servant for Chelsea.

So when you analyse the performance of David Luiz against Manchester United, which can be best described as totally erratic, it only serves to highlight the problems Chelsea face when it comes to replacing Terry.

With Terry suspended for the United game, Roberto di Matteo went with Luiz and Gary Cahill at centre half but, once again, Luiz looked like he had a mistake in him right from the kick-off and Cahill was 10 times better than him by comparison.

In the final 20 minutes of the game, Luiz was like something out of a comedy strip with his farcical defending and ridiculous appetite for barging into Robin van Persie.

When the game is as tense and volatile as it was in the latter stages at Stamford Bridge, the last thing you want is your team-mates to be putting you under pressure.

But Luiz was fortunate not to be sent off as his display became more and more erratic and there is nothing worse for the rest of the team to have a player alongside you who you have doubts about. Right from the second minute, when he was turned by Van Persie, it was clear that Luiz was going to have one of those days.

He was unlucky to score an own goal to give United the lead because the ball simply bounced off the post and on to his back before finding the net, but he was certainly culpable for the second and third goals.

Like many defenders today, Luiz tries to block with his hands behind his back. That is understandable when you are inside the penalty area, but the Brazilian was a long way outside the 18-yard box when he attempted to block Antonio Valencia’s cross in the same manner and the United player just brushed past him.

It was so casual by Luiz, allowing Valencia to put the ball through his legs, but his part in United’s third goal was just as bad, if not worse. He attempted to make two blocks with his hands behind back, but the ball goes through his legs again and Javier Hernandez scores the winning goal.

In a week when all the talk has been about systems and different ways of defending, on the back of Micah Richards questioning Manchester City's defensive tactics under Roberto Mancini, the reality of defending properly has been lost amid the debate.

The concept of defending properly is the same as it has always been and it is nothing to do with systems – it is all about positioning.

If you get four defenders in the right position, it can be incredibly difficult for the opposing team to get through, but if the defenders are all over the place, then it becomes a gravy train for the opposition, with chances coming along one after the other.

And when you have a player such as Luiz in the back four, with nobody knowing what or where he is going to be next, it creates uncertainty.

It is almost two years since he arrived at Chelsea, but he has rarely gone more than four or five games without making a mistake.

In fairness to him, it is not easy to settle into a new back four. When Mancini said last week that good players can play in any system, it really isn’t that simple.

All players need time to adjust to new team-mates, a new club or a new system devised by the manager. A partnership may look poor after three or four games, but give it 10, 12 or 14 games and you might just be in business.

The problem with Luiz is that he has never looked like going on a run of 20 games without making a mistake, even when he has played alongside Terry.

It is an issue for Di Matteo to deal with, but for some reason, whenever Terry is available, Chelsea tend to go with Terry and Luiz at the back, with Cahill missing out.

If Chelsea are to be successful this season, though, Di Matteo has a big decision to make about the centre of his defence, but the bigger one comes when they have to replace Terry who, due to age and injury, is coming towards the end.

United may also have given Chelsea another problem to deal with when it comes to protecting their fullbacks.

Although the attacking trio of Eden Hazard, Oscar and Juan Mata have been magnificent this season, their presence in the team leaves Chelsea so vulnerable down the flanks because there is nobody around to stop the fullbacks being exposed.

By playing with two wingers, United were able to attack in numbers down both sides of the pitch and the tactic paid off.

But it was the centre of their defence that caused Chelsea most problems and Luiz was central to that.